The present invention relates to a sonar system with a transmitting arrangement for simultaneously pivoting or sweeping a plurality of transmitting directional characteristics in adjacent sectors of an observation range.
Sonar systems in which a high transmitting level of sound is obtained by bundling the transmitting power emitted by a group of electroacoustical transducers into a narrow directional characteristic which is pivoted or swept, intermittently or continuously, over the observation area of interest are known in the art. This method of sound transmission is referred to in the art literature as the rotational directional transmission or RDT method.
In an existing sonar system employing this principle, three transmitting directional characteristics are produced which are offset with respect to one another by 120.degree. each in the manner of a star and which emit sound while being simultaneously swept over a respective sector of 120.degree. so that the entire circle of 360.degree. is covered.
The advantage of the RDT method is that bundling of the transmitting power results in a high transmission level -- and thus a correspondingly wide range -- and that at the same time a wide observation sector can be sounded within a short period of time.
To achieve these advantages, several drawbacks must be taken into account in the RDT method. Since the transmitting directional characteristics cover their assigned sector in a finite time period, echo pulses from different equidistant targets distributed over a sector arrive with a delay determined by the pivoting or sweeping speed. These targets are then displayed on the conventional PPI (Plan Position Indicator) display not on a circular arc but with a spiral-shaped distortion corresponding to the differing arrival times of the echo pulses.
This spiral-shaped distoration has a particularly unfavorable effect in an RDT method system utilizing a plurality of simultaneously transmitting directional characteristics when a target lies in the overlapping range of two adjacent transmitting directional characteristics. In such a system, a target so located will be displayed by two separately illuminated dots, one lying behind the other with the inner dot being produced by an echo from one of the transmitting directional characteristics at the beginning of the transmitting period and the outer dot by an echo from the adjacent characteristic at the end of the transmitting period.
To compensate for the distance errors caused by the finite duration of the pivoting movement, special compensation devices are required. This compensation, however, becomes particularly difficult due to the above-described double display of a target disposed in a zone of overlap between two adjacent transmitting sectors.
Such double displays can be avoided, as proposed in German Pat. application P 17 66 751.5 or U.S. Pat. application Ser. No. 842,112 filed July 16, 1969, now Pat. No. 3,609,673 by providing a stepwise switching or sweep direction for the transmitting directional characteristics within their respective assigned transmitting sector.
Each transmitting sector is here divided into n partial sectors each of which has the width of the transmitting directional characteristic. Within each transmitting period the transmitting directional characteristics are now switched, beginning in the vicinity of their center position at n/2, in rapid succession to n/2 + 1, n/2 - 1, n/2 + 2 . . . until they reach the edges of their respective sectors. Coarse double displays are thus avoided and the PPI image becomes almost unambiguous because double echoes can have only very slight distances from one another.
One particular drawback, however, of such a system, which drawback is inherent to all irregularly pivoting or sweeping transmitting methods in the ranging art, is that they radiate a relatively broad spectrum whose half-value width corresponds to the inverse of the period of dwell of the transmitting directional characteristic in one direction.